They had several rounds of negotiations in the first month of the war, but nothing came out of them. Ukraine doesn't want Russia to get any territory whatsoever, not even Crimea. So, now Russia is taking it by force. Recently they're not even trying anymore to have negotiations because they already know it would be a waste of time with neither side agreeing

They're not going to stop until they feel like they achieved their objectives in Ukraine. They wanted to secure a water and land bridge to Crimea, and they did it. Now they are focusing on taking the Donbass and Luhansk oblasts